Surface modification of silicon-containing electrodes using carbon dioxide

ABSTRACT

Various implementations of a method of forming an electrochemical cell include providing a first electrode, a second electrode, a separator between the first and second electrodes, and an electrolyte in a cell container. The first electrode can include silicon-dominant electrochemically active material. The silicon-dominant electrochemically active material can include greater than 50% silicon by weight. The method can also include exposing at least a part of the electrochemical cell to CO 2 , and forming a solid electrolyte interphase (SEI) layer on the first electrode using the CO 2 .

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet as filed with the presentapplication are hereby incorporated by reference under 37 CFR 1.57.

BACKGROUND Field

The present application relates generally to silicon-containing batteryelectrodes. In particular, the present application relates to electrodesincluding silicon particles for use in battery electrodes.

Description of the Related Art

A lithium-ion (Li-ion) battery typically includes a separator and/orelectrolyte between an anode and a cathode. In one class of batteries,the separator, cathode and anode materials are individually formed intosheets or films. Sheets of the cathode, separator and anode aresubsequently stacked or rolled with the separator separating the cathodeand anode (e.g., electrodes) to form the battery. Typical electrodesinclude electro-chemically active material layers on electricallyconductive metals (e.g., aluminum and copper). Films can be rolled orcut into pieces which are then layered into stacks. The stacks are ofalternating electro-chemically active materials with the separatorbetween them.

When an anode is accepting lithium, the surface of the anode is in areducing state. At this potential, carbonate electrolytes used inlithium ion batteries may react on the anode surface forming a solidelectrolyte interface (SEI). The anode SEI composition may play animportant role in maintaining the integrity of the anode surface andelectrolyte when cycling lithium ion batteries. Silicon anodes expandand contract during cycling which can break the SEI layer and causedegradation of the electrolyte onto the newly exposed anode surface andcan cause further impedance growth. There is a need to improve the SEIlayer formed on a silicon anode to enable longer term cycle life.

SUMMARY

In certain implementations, a method of forming an electrochemical cellis provided. The method can include providing a first electrode, asecond electrode, a separator between the first and second electrodes,and an electrolyte in a cell container. The first electrode can comprisesilicon-dominant electrochemically active material. The silicon-dominantelectrochemically active material can comprise greater than 50% siliconby weight. The method can also include exposing at least a part of theelectrochemical cell to CO₂ and forming a solid electrolyte interphase(SEI) layer on the first electrode using the CO₂.

In some instances, the SEI layer can comprise solid carbonate on LiFparticles. In some methods, forming the SEI layer can occur duringlithiation.

In some instances, the silicon-dominant electrochemically activematerial can comprise the silicon at about 60% to about 100% by weight.In some instances, the silicon-dominant electrochemically activematerial can comprise the silicon at about 70% to about 100% by weight.

In some implementations, the second electrode can comprise nickel cobaltaluminum (NCA). In some such examples, the CO₂ can be produced fromLi₂CO₃ on a surface of the NCA. In some implementations, the secondelectrode can comprise lithium cobalt oxide (LCO), lithium manganeseoxide (LMO), or nickel manganese cobalt (NMC).

In some instances, the CO₂ can be provided in the cell container. Forexample, the CO₂ can be provided in an amount of about 0.01 to about 0.6mg per mg of the silicon. As another example, the CO₂ can be provided inan amount of about 0.1 to about 0.6 mg per mg of the silicon. In someimplementations, the CO₂ can be provided by adding dry ice to theelectrolyte. In some implementations, the CO₂ can be dissolved in theelectrolyte. As an example, the CO₂ can be dissolved in an amount ofabout 0.01 to about 0.6 mg per mg of the silicon. In someimplementations, the CO₂ can be produced from a carbonate in the cellcontainer.

In some implementations, the electrolyte can comprise greater than about10% to about 90% carbonate solvent. In some instances, the electrolytecan comprise a cyclic carbonate solvent. For example, the solvent cancomprise fluoroethylene carbonate (FEC), difluoroethylene carbonate(F2EC), trifluoroethylene carbonate (F3EC), ethylene carbonate (EC),propylene carbonate (PC), trifluoropropylene carbonate (TFPC), or amixture thereof. In some instances, the electrolyte can comprise alinear carbonate solvent. For example, the solvent can compriseethylmethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate(DMC), methyl 2,2,2-trifluoroethyl carbonate (TFEMC), ethyl2,2,2-trifluoroethyl carbonate (TFDEC), difluoromethyl fluoromethylcarbonate (TFDMC), or a mixture thereof.

In some implementations, the method can further comprise holding thefirst electrode, the second electrode, and the separator together whilethe at least a part of the electrochemical cell is exposed to CO₂. Insome instances, the separator can comprise polyolefin, cellulose, or acombination thereof. The first electrode, the second electrode, and theseparator can be held together with an adhesive. For example, theadhesive can comprise poly(vinylidene fluoride)/poly(methylmethacrylate) (PVDF/PMMA). The first electrode, the second electrode,and the separator can be held together by a gel electrolyte. The firstelectrode, the second electrode, and the separator can be held togethermechanically. For example, the first electrode, the second electrode,and the separator can be held together with at least one clamp, bolt,clip, or combination thereof.

In some instances, the first electrode can be an anode and the secondelectrode can be a cathode. The first electrode can comprise aSi-composite electrochemically active film. The first electrode can be aself-supported film.

In certain implementations, an electrochemical device can be provided.The device can include a first electrode, a second electrode, aseparator between the first electrode and the second electrode, andelectrolyte in contact with the first electrode, the second electrode,and the separator. The first electrode can include silicon-dominantelectrochemically active material. The silicon-dominantelectrochemically active material can comprise greater than 50% siliconby weight. The first electrode can also include a solid electrolyteinterphase (SEI) layer. The SEI layer can comprise solid carbonate onLiF particles.

In some instances, the electrochemically active material can comprisethe silicon at about 60% to about 100% by weight. In some instances, theelectrochemically active material can comprise the silicon at about 70%to about 100% by weight.

In some devices, the second electrode can comprise nickel cobaltaluminum (NCA). In some devices, the second electrode can compriselithium cobalt oxide (LCO), lithium manganese oxide (LMO), or nickelmanganese cobalt (NMC).

In some devices, the electrolyte can comprise greater than about 10% toabout 90% carbonate solvent. The electrolyte can comprise a cycliccarbonate solvent. For example, the solvent can comprise fluoroethylenecarbonate (FEC), difluoroethylene carbonate (F2EC), trifluoroethylenecarbonate (F3EC), ethylene carbonate (EC), propylene carbonate (PC),trifluoropropylene carbonate (TFPC), or a mixture thereof. Theelectrolyte can comprise a linear carbonate solvent. For example, thesolvent can comprise ethylmethyl carbonate (EMC), diethyl carbonate(DEC), dimethyl carbonate (DMC), methyl 2,2,2-trifluoroethyl carbonate(TFEMC), ethyl 2,2,2-trifluoroethyl carbonate (TFDEC), difluoromethylfluoromethyl carbonate (TFDMC), or a mixture thereof.

In some devices, the separator can comprise polyolefin, cellulose, or acombination thereof. The first electrode, the second electrode, and theseparator can be held together with an adhesive. For example, theadhesive can comprise poly(vinylidene fluoride)/poly(methylmethacrylate) (PVDF/PMMA). In some instances, the electrolyte cancomprise a gel electrolyte.

In some devices, the first electrode can be an anode, and the secondelectrode can be a cathode. The first electrode can comprise aSi-composite electrochemically active film. In some instances, the firstelectrode can be a self-supported film. In some instances, theelectrochemical device can be a battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an example method of forming an electrochemical cell inaccordance with certain embodiments described herein.

FIG. 2A shows the discharge capacity retention as a function of cyclesof cells with and without CO₂ added.

FIG. 2B shows the discharge capacity retention as a function of cyclesof cells with and without CO₂ added.

FIG. 2C shows the dQ/dV during the first formation charge as a functionof voltage of cells with and without CO₂ added.

FIG. 2D shows the cell thickness as a function of cycles of cells withand without CO₂ added.

FIG. 3 shows the discharge capacity retention as a function of cycles ofcells with and without CO₂ added.

FIG. 4A shows the dQ/dV during the first formation charge as a functionof voltage of cells with and without CO₂ added.

FIG. 4B shows the discharge capacity retention as a function of cyclesof cells with and without CO₂ added.

FIG. 4C shows the discharge capacity retention as a function of cyclesof cells with and without CO₂ added.

DETAILED DESCRIPTION

This application describes certain embodiments of electrodes (e.g.,anodes and cathodes) comprising silicon material as electrochemicallyactive material for use in electrochemical cells. Silicon can be apotentially high energy per unit volume host material, such as forlithium ion batteries. For example, silicon has a high theoreticalcapacity and can increase the energy density of lithium ion batteriescompared with lithium ion batteries using other active materials such asgraphite. However, silicon can swell in excess of 300% upon lithiuminsertion. Accordingly, batteries with silicon anodes may exhibit morerapid capacity loss upon cycling compared with batteries with graphiteanodes. The repeat expansion and contraction of silicon particles duringcharge and discharge can lead to mechanical failure of the anode duringcycling. Mechanical failure can expose new surfaces of silicon which canreact with the electrolyte, irreversibly incorporating Li-ions in thesolid electrolyte interface/interphase (SEI) and leading to capacityloss. Certain embodiments described herein can include silicon materialwith a modified surface, leading to improved cycling performance. Forexample, some embodiments can provide an SEI with increased stability(e.g., a substantially stable SEI and/or a stable SEI in some instances)to improve the capacity retention and reduce (e.g., and/or prevent insome instances) fast fading.

Some implementations described herein can create an SEI layer on asilicon-based electrode by introducing carbon dioxide (CO₂) duringformation and cycling. Electrochemical cells with electrodes formed withsuch an SEI layer described herein may have advantages includingincreased cycle life, increased energy density, improved safety andcost, and/or decreased electrolyte consumption. Without being bound bytheory (e.g., during lithiation in the presence of fluorine and CO₂),LiF particles can be formed and carbonate species may act as a cappingagent on the surface of an electrode. The carbonates in currentelectrolyte systems do not efficiently form a protective layer on theLiF to control the LiF particle size. CO₂ can be more efficient atforming solid carbonates such that the LiF particles do not grow to alarger size. Smaller LiF particles in the SEI layer can allow for a moreuniform distribution on the electrode surface, yielding a more lithiumconductive SEI layer.

In some implementations, the electrodes treated with CO₂ can includesilicon particles mixed with binder in a solvent and coated onto acurrent collector. For example, the Si particles may be used as theactive material in powder form. The binder and current collector may beany of those known in the art or yet to be developed. For example, thecurrent collector may be a copper or nickel foil. In some instances,conductive particles (e.g., carbon) may be added.

In some implementations, the electrodes treated with CO₂ can befilm-based electrodes. For example, some composite material films can bemonolithic, self-supporting structures using pyrolyzed polymer, e.g., asdescribed in U.S. patent application Ser. No. 13/008,800 (U.S. Pat. No.9,178,208), entitled “Composite Materials for Electrochemical Storage;”U.S. patent application Ser. No. 13/601,976, filed Aug. 31, 2012, andpublished on Jun. 19, 2014 as U.S. Patent Application Publication No.2014/0170498, entitled “Silicon Particles for Battery Electrodes;” orU.S. patent application Ser. No. 13/799,405 (U.S. Pat. No. 9,553,303),entitled “Silicon Particles for Battery Electrodes,” each of which isincorporated by reference herein. In some embodiments, theself-supported composite material film can be used as an electrode(e.g., without a current collector). In some instances, electrodes withsilicon particles in carbonized polymer matrix (e.g., compared toelectrodes with silicon particles suspended in a polymeric binder), canallow for a different surface for formation of the SEI layer as thesurface of silicon would be initially covered in carbon. For example, insome electrodes with silicon particles in a carbonized polymer matrix,the carbon may cover substantially all of the silicon surface while mostpolymeric binders typically do not cover all of the active materialsurface.

Some composite material films may be provided on a current collector toform an electrode. In some embodiments, the composite material film canbe attached to a current collector using an attachment substance. Theattachment substance and current collector may be any of those known inthe art or yet to be developed. For example, some composite materialfilms can be provided on a current collector as described in U.S. patentapplication Ser. No. 13/333,864 (U.S. Pat. No. 9,397,338), entitled“Electrodes, Electrochemical Cells, and Methods of Forming Electrodesand Electrochemical Cells;” or U.S. patent application Ser. No.13/796,922 (U.S. Pat. No. 9,583,757), entitled “Electrodes,Electrochemical Cells, and Methods of Forming Electrodes andElectrochemical Cells, each of which is incorporated by referenceherein. Some composite material films may be formed on a currentcollector, e.g., as described in U.S. patent application Ser. No.15/471,860, filed Mar. 28, 2017, and published on Oct. 4, 2018 as U.S.Patent Application Publication No. 2018/0287129, entitled “Methods ofForming Carbon-Silicon Composite Material on a Current Collector,” whichis incorporated by reference herein.

FIG. 1 shows an example method of forming an electrochemical cell. Themethod 100 can include providing a first electrode, a second electrode,a separator between the first and second electrodes, and an electrolytein a cell container, as shown in block 110, and exposing at least a partof the electrochemical cell (e.g., the first electrode) to CO₂, as shownin block 120. The method 100 can also include forming a solidelectrolyte interphase (SEI) layer on the first electrode (e.g., usingthe CO₂), as shown in block 130.

With respect to block 110, the first electrode can comprise asilicon-containing electrode. In certain embodiments, the siliconmaterial can be at least partially crystalline, substantiallycrystalline, and/or fully crystalline. Furthermore, the silicon materialmay be substantially pure silicon or may be a silicon alloy. In oneembodiment, the silicon alloy includes silicon as the primaryconstituent along with one or more other elements.

In some embodiments, the silicon material can comprise siliconpowders/particles. For example, the particle size (e.g., diameter or alargest dimension of the silicon particles) can be less than about 50μm, less than about 40 μm, less than about 30 μm, less than about 20 μm,less than about 10 μm, less than about 1 μm, between about 10 nm andabout 50 μm, between about 10 nm and about 40 μm, between about 10 nmand about 30 μm, between about 10 nm and about 20 μm, between about 0.1μm and about 50 μm, between about 0.1 μm and about 40 μm, between about0.1 μm and about 30 μm, between about 0.1 μm and about 20 μm, betweenabout 0.5 μm and about 50 μm, between about 0.5 μm and about 40 μm,between about 0.5 μm and about 30 μm, between about 0.5 μm and about 20μm, between about 1 μm and about 50 μm, between about 1 μm and about 40μm, between about 1 μm and about 30 μm, between about 1 μm and about 20μm, between about 1 μm and about 15 μm, between about 1 μm and about 10μm, between about 10 nm and about 10 μm, between about 10 nm and about 1μm, less than about 500 nm, less than about 100 nm, about 100 nm, etc.All, substantially all, or at least some of the silicon particles maycomprise the particle size (e.g., diameter or largest dimension)described above. For example, an average particle size (or the averagediameter or the average largest dimension) or a median particle size (orthe median diameter or the median largest dimension) of the siliconparticles can be less than about 50 μm, less than about 40 μm, less thanabout 30 μm, less than about 20 μm, less than about 10 μm, less thanabout 1 μm, between about 10 nm and about 50 μm, between about 10 nm andabout 40 μm, between about 10 nm and about 30 μm, between about 10 nmand about 20 μm, between about 0.1 μm and about 50 μm, between about 0.1μm and about 40 μm, between about 0.1 μm and about 30 μm, between about0.1 μm and about 20 μm, between about 0.5 μm and about 50 μm, betweenabout 0.5 μm and about 40 μm, between about 0.5 μm and about 30 μm,between about 0.5 μm and about 20 μm, between about 1 μm and about 50μm, between about 1 μm and about 40 μm, between about 1 μm and about 30μm, between about 1 μm and about 20 μm, between about 1 μm and about 15μm, between about 1 μm and about 10 μm, between about 10 nm and about 10μm, between about 10 nm and about 1 μm, less than about 500 nm, lessthan about 100 nm, about 100 nm, etc. In some embodiments, the siliconparticles may have a distribution of particle sizes. For example, atleast about 95%, at least about 90%, at least about 85%, at least about80%, at least about 70%, or at least about 60% of the particles may havethe particle size described herein.

With continued reference to block 110, the first electrode can includefrom greater than 0% to about 100% by weight of silicon. For example,the amount of silicon by weight of the electrode (or theelectrochemically active material) can include any weight percent withinthis range (e.g., about 10%, about 15%, about 20%, about 25%, about 30%,about 35%, about 40%, about 45%, about 50%, about 55%, about 60%, about65%, about 70%, about 75%, about 80%, about 85%, about 90%, about 95%,about 96%, about 97%, about 98%, about 99%, about 100%, etc.), or anyrange within this range such as any range formed by the example values(e.g., greater than about 0% to about 25% by weight, greater than about0% to about 35% by weight, greater than about 0% to about 50% by weight,greater than about 0% to about 70% by weight, greater than about 0% toabout 90% by weight, greater than about 0% to about 95% by weight,greater than about 0% to about 99% by weight, from about 10% to about35% by weight, from about 10% to about 50% by weight, from about 10% toabout 90% by weight, from about 10% to about 95% by weight, from about10% to about 99% by weight, from about 10% to about 100% by weight, fromabout 30% to about 85% by weight, from about 30% to about 90% by weight,from about 30% to about 95% by weight, from about 30% to about 99% byweight, from about 30% to about 100% by weight, from about 50% to about85% by weight, from about 50% to about 90% by weight, from about 50% toabout 95% by weight, from about 50% to about 99% by weight, from about50% to about 100% by weight, from about 60% to about 85% by weight, fromabout 60% to about 90% by weight, from about 60% to about 95% by weight,from about 60% to about 99% by weight, from about 60% to about 100% byweight, from about 70% to about 85% by weight, from about 70% to about90% by weight, from about 70% to about 95% by weight, from about 70% toabout 99% by weight, from about 70% to about 100% by weight, from about80% to about 90% by weight, from about 80% to about 95% by weight, fromabout 80% to about 99% by weight, from about 80% to about 100% byweight, etc.).

In some instances, the first electrode can include a silicon-dominantelectrode. For example, the first electrode can include silicon-dominantelectrochemically active material. As an example, the electrochemicallyactive material can include at least about 50% to about 95% by weight ofsilicon, at least about 50% to about 97% by weight of silicon, at leastabout 50% to about 99% by weight of silicon, at least about 50% to about100% by weight of silicon, at least about 60% to about 95% by weight ofsilicon, at least about 60% to about 97% by weight of silicon, at leastabout 60% to about 99% by weight of silicon, at least about 60% to about100% by weight of silicon, at least about 70% to about 95% by weight ofsilicon, at least about 70% to about 97% by weight of silicon, at leastabout 70% to about 99% by weight of silicon, at least about 70% to about100% by weight of silicon, at least about 80% to about 95% by weight ofsilicon, at least about 80% to about 97% by weight of silicon, at leastabout 80% to about 99% by weight of silicon, at least about 80% to about100% by weight of silicon, at least about 90% to about 95% by weight ofsilicon, at least about 90% to about 97% by weight of silicon, at leastabout 90% to about 99% by weight of silicon, at least about 90% to about100% by weight of silicon, at least about 95% to about 99% by weight ofsilicon, at least about 95% to about 97% by weight of silicon, at leastabout 95% to about 100% by weight of silicon, etc. In some examples, theelectrochemically active material can include 100% silicon.

As described herein, the first electrode can include the silicon powderscoated on a current collector. For example, the silicon powders can becoated on a current collector with a binder. For instance, the powderscan be added to a slurry and coated on a current collector. Additionalconductive particles (e.g., graphite) can also be added to the slurry.

In some embodiments, the first electrode can be a film-based electrode.For example, the electrode material can be formed into silicon-carboncomposite films fabricated through using the silicon material andcarbonized polymer (e.g., a hard carbon). In various embodiments, thefilm-based electrodes may be self-supported structures or attached to acurrent collector. When attached to a current collector, an attachmentsubstance can be applied using a solution (e.g., a wet process) orapplied as in a substantially solid state (e.g., a substantially dryprocess). In some embodiments, the electrode can comprise graphite. Insome embodiments, the electrode can comprise glass carbon (e.g., a hardcarbon prepared from carbonization of a polymer). The current collectorcan be any known in the art or yet to be developed. In some instances,the current collector can comprise a Cu or Ni foil.

In some instances, the first electrode can be an anode and the secondelectrode can be a cathode. The second electrode can include nickelcobalt aluminum (NCA), lithium cobalt oxide (LCO), lithium manganeseoxide (LMO), or nickel manganese cobalt (NCM). However, the secondelectrode is not particularly limited. For example, the second electrodecan be any cathode known in the art or yet to be developed. In someembodiments, the separator can include polyolefin, cellulose, or acombination thereof. However, the separator is not particularly limited.For example, the separator can be any of those known in the art or yetto be developed. The electrolyte can include a carbonate solvent, suchas a cyclic carbonate solvent or a linear carbonate solvent. However,the electrolyte is not particularly limited. For example, theelectrolyte can be any of those known in the art or yet to be developed.The cell container can include a housing, casing, a pouch, etc. However,the cell container is not particularly limited. For example, the cellcontainer can be any of those known in the art or yet to be developed.

With respect to block 120, the method can comprise exposing at least apart of the electrochemical cell (e.g., the first electrode) to CO₂. TheCO₂ can be provided to at least a portion of the electrochemical cell orcan be produced by at least a portion of the electrochemical cell. Insome instances, the CO₂ can be provided in the cell container. Forexample, about 0.01 mg to about 0.6 mg (e.g., about 0.03 mg to about 0.6mg, about 0.05 mg to about 0.6 mg, about 0.1 mg to about 0.6 mg, about0.15 mg to about 0.6 mg, about 0.2 mg to about 0.6 mg, about 0.25 mg toabout 0.6 mg, about 0.3 mg to about 0.6 mg, about 0.35 mg to about 0.6mg, about 0.4 mg to about 0.6 mg, about 0.45 mg to about 0.6 mg, about0.5 mg to about 0.6 mg, etc.) of CO₂ per mg of silicon can be providedin the cell container. In some instances, the CO₂ can be added from aCO₂ gas source or from dry ice. The CO₂ can be provided in the cellcontainer during the electrolyte injection step. The CO₂ source can beadded to the cell stack or to the electrolyte. In some instances, theCO₂ can be dissolved (e.g., directly dissolved) in the electrolyte. Forexample, about 0.01 mg to about 0.6 mg of CO₂ per mg of silicon can bedissolved in the electrolyte. As one example, dry CO₂ gas (e.g., 0.01mg, 0.015 mg, or 0.02 mg CO₂ per mg of silicon at room temperature and 1atm) can be bubbled through the electrolyte solution.

Additionally or alternatively, the CO₂ can be produced from a carbonateor other additive in the cell container. In some implementations, theCO₂ can be produced by the second electrode. For instance, the secondelectrode can comprise NCA and the CO₂ can be produced by Li₂CO₃ (e.g.,impurity) on the surface of the NCA. For example, CO₂ can be produced bythe reaction 2HF+Li₂CO₃=2LiF+H₂O+CO₂. In some implementations, the CO₂can be produced by the electrolyte. For instance, the electrolyte canhave greater than about 10% to about 90% (e.g., about 20% to about 90%,about 30% to about 90%, about 40% to about 90%, about 50% to about 90%,about 60% to about 90%, about 70% to about 90%, about 80% to about 90%)carbonate solvent. In some examples, the carbonate solvent can comprisea cyclic carbonate solvent such as fluoroethylene carbonate (FEC),difluoroethylene carbonate (F2EC), trifluoroethylene carbonate (F3EC),ethylene carbonate (EC), propylene carbonate (PC), trifluoropropylenecarbonate (TFPC), or a mixture thereof. In some examples, the carbonatesolvent can comprise a linear carbonate solvent, such as ethylmethylcarbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC),methyl 2,2,2-trifluoroethyl carbonate (TFEMC), ethyl2,2,2-trifluoroethyl carbonate (TFDEC), difluoromethyl fluoromethylcarbonate (TFDMC), or a mixture thereof.

With reference to block 130, exposure to CO₂ can form an SEI layer onthe first electrode, such as during lithiation (e.g., during formationand/or cycling). In some implementations, the SEI layer can includesolid carbonate on LiF particles. Lithium salts may produce carbonate(or oxalate) capping agents. As described herein, the solid carbonatecan provide a cap on the LiF particles to limit its growth duringformation, allowing smaller particles to form a more uniformdistribution in the SEI layer on the electrode surface. In variousimplementations, the SEI layer on the electrode can be about 1 nm toabout 20 nm thick (e.g., about 1 nm to about 15 nm, about 1 nm to about10 nm, about 3 nm to about 20 nm, about 3 nm to about 15 nm, about 3 nmto about 10 nm, about 4 nm to about 20 nm, about 4 nm to about 15 nm,about 4 nm to about 10 nm, about 5 nm to about 20 nm, about 5 nm toabout 15 nm, about 5 nm to about 10 nm, etc.).

In some instances, the method 100 can further include holding the firstelectrode, the second electrode, and the separator together while atleast a portion of the electrochemical cell is exposed to CO₂. Withoutbeing bound by theory, the first electrode, the second electrode, andthe separator can be kept together during formation and cycling asgenerated or added gas may degrade the cell otherwise. Holding the celllayers together can reduce and/or prevent CO₂ gas from impedinginterlayer contact, which can reduce and/or prevent degradation due todelamination. As an example, the first electrode, the second electrode,and the separator can be held together with a gel electrolyte, such aselectrolyte with poly(vinylidene fluoride) (PVDF), poly(ethylene oxide)(PEO), poly(methyl methacrylate) (PMMA), polyacrylonitrile (PAN),polyethylene glycol (PEG), etc. As another example, the first electrode,the second electrode, and the separator can be held together with anadhesive layer on one or more of the electrodes and/or separator and onone or more sides. For example, the adhesive can comprisepoly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/PMMA). In someimplementations, the separator can include polyolefin, cellulose, or acombination thereof, and the separator can be coated with the adhesive.After the first electrode, the second electrode, and the separator areheld together, they (or the entire battery) can be pressed. In someinstances, they can be pressed with heat.

In some implementations, the first electrode, the second electrode, andthe separator can be held together mechanically. The first electrode,the second electrode, and the separator can be held together withpressure. In some instances, the first electrode, the second electrode,and the separator can be held together with a clamp, bolt, clip, orcombination thereof. For example, the first electrode, the secondelectrode, and the separator can be clamped between plates. The platescan be held together with one or more bolts or clips. Other examples arepossible. After formation, the clamp, bolt, clip, or combination thereofcan be removed or can remain (or be replaced with a different mechanicaldevice) for use during operation.

Various implementations of an electrochemical device can comprise afirst electrode, a second electrode, and a separator between the firstand second electrodes. The electrochemical device can also include anelectrolyte in contact with the first electrode, the second electrode,and the separator. The first electrode can include silicon-comprisingelectrochemically active material. In some instances, the firstelectrode can include silicon-dominant electrochemically active materialcomprising greater than 50% (e.g., about 60% to about 100%, about 70% toabout 100%, about 80% to about 100%, about 90% to about 100%, about 95%to about 100%, etc.) silicon by weight. The first electrode can alsoinclude an SEI layer comprising solid carbonate on LiF particles. Thefirst electrode can comprise a silicon-composite electrochemicallyactive film. In some instances, the first electrode is a self-supportedfilm. The electrochemical device can be a battery, e.g., a Li-ionbattery.

EXAMPLES Example 1

Cells were built with NCM cathodes and silicon-containing anodes. Cellswere filled with either 1.2M LiPF₆ in EC/EMC (3/7 wt %) or 1.2M LiPF₆ inFEC/PC/EMC (2/1/7 wt %). The cells were hot/cold pressed to activate thepolymer adhesive on the separator in order to maintain adhesion betweenthe anode, cathode, and separator layers, even when the cell expandedfrom adding CO₂. CO₂ was added in the form of dry ice, directly to thecell stack. Cells were left overnight and tested the next day.

The Si-dominant anodes contained about 80 wt % Si, 5 wt % graphite and15 wt % glassy carbon (which could be derived from resin) and werelaminated on 15 μm Cu foil. The average loading was about 3.8 mg/cm².The cathodes contained about 92 wt % NCM-622, 4 wt % Super P and 4 wt %PVDF, and were coated on 15 μm Al foil. The average loading was about 23mg/cm².

FIG. 2A shows the discharge capacity retention as a function of cyclesof the cells with 1.2M LiPF₆ in FEC/PC/EMC (2/1/7 wt %); FIG. 2B showsthe discharge capacity retention as a function of cycles of the cellswith 1.2M LiPF₆ in EC/EMC (3/7 wt %); FIG. 2C shows the charge capacity(dQ/dV—During Charge) of the cells as a function of voltage; and FIG. 2Dshows the cell thickness during cycle life. The solid lines are theresults of the control cells without CO₂ and the broken lines are theresults of the cells with CO₂ added. The discharge capacity curves inFIGS. 2A-2B show that the cycle life is improved when CO₂ is added tothe cells.

The dQ/dV data for both control and CO₂ additive-containing cells wereobtained through the following testing protocol: Rest 5 minutes, chargeat 0.025 C to 25% nominal capacity, charge at 0.2 C to 4.2 V until 0.05C, rest 5 minutes, discharge at 0.2 C to 3.0 V, rest 5 minutes. The 1stformation cycle dQ/dV curves in FIG. 2C show that CO₂additive-containing cells show a strong peak between 1.5-2.0V whichindicates that the SEI layer is changed and may passivate furtherreaction of electrolytes at higher voltages. The cell thickness increasefor cells with CO₂ is reduced compared to the control cells, as shown inFIG. 2D.

Example 2

Cells were built with NCM cathodes and silicon-containing anodes. Cellswere filled with 1.2M LiPF₆ in FEC/EMC (3/7 wt %). The cells werehot/cold pressed to activate the polymer adhesive on the separator inorder to maintain adhesion between the anode, cathode, and separatorlayers, even when the cell expanded from adding CO₂. CO₂ was added inthe form of dry ice, directly to the cell stack. Cells were clamped withmetal plates and bolts to simulate the pressure applied in an ElectricVehicle (EV) battery pack housing, then tested.

The Si-dominant anodes contained about 80 wt % Si, 5 wt % graphite and15 wt % glassy carbon (which could be derived from resin) and werelaminated on 15 μm Cu foil. The average loading was about 3.8 mg/cm².The cathodes contained about 92 wt % NCM-622, 4 wt % Super P and 4 wt %PVDF and were coated on 15 μm Al foil. The average loading was about 23mg/cm². Cells were clamped with stainless steel clamps with a fixed gapof the measured initial cell thickness and were held together with 4bolts.

FIG. 3 shows the discharge capacity retention of cells with and withoutCO₂ where solid lines are the results of the control cells without CO₂and the broken lines are the results of the cells with CO₂ added. Thedischarge capacity curves shown in FIG. 3 show that the cycle life isimproved when CO₂ is added to the cells.

Example 3

Cells were built with silicon-containing anodes and either NCM or NCAcathodes. Cells were filled with 1.2M LiPF₆ in FEC/EMC (3/7 wt %). Thecells were hot/cold pressed to activate the polymer adhesive on theseparator in order to maintain adhesion between the anode, cathode, andseparator layers, even when the cell expanded from adding CO₂. CO₂ wasadded in the form of dry ice, directly to the cell stack. Cells wereclamped with acrylic plates and binder clips to simulate appliedpressure in an EV battery pack housing, then tested.

The Si-dominant anodes contained about 80 wt % Si, 5 wt % graphite and15 wt % glassy carbon (which could be derived from resin), and werelaminated on 15 μm Cu foil. The average loading was about 3.8 mg/cm².The cathodes contained about 92 wt % NCM-622 or NCA, 4 wt % Super P and4 wt % PVDF, and were coated on 15 μm Al foil. The average loading wasabout 23 mg/cm². Cells were clamped with acrylic clamps held togetherwith large binder clips.

FIG. 4A shows the charge capacity (dQ/dV) as a function of voltage ofthe cells with NCA cathodes; FIG. 4B shows the discharge capacityretention as a function of cycles of the cells with the NCA cathodes;FIG. 4C shows the discharge capacity retention as a function of cyclesof the cells with the NCM cathodes, where solid lines are the results ofthe control cells without CO₂ and the broken lines are the results ofthe cells with CO₂ added.

The dQ/dV data for both control and CO₂ additive-containing cells wereobtained through the following testing protocol: Rest 5 minutes, chargeat 0.025 C to 25% nominal capacity, charge at 0.2 C to 4.2 V until 0.05C, rest 5 minutes, discharge at 0.2 C to 3.0 V, rest 5 minutes. The 1stformation cycle dQ/dV curves in FIG. 4A indicate that there issubstantially no change to the anode SEI formation when NCA is used. Thepeak seen in FIG. 2C when CO₂ is introduced already exists with NCA evenwhen no CO₂ is introduced. Thus, there is little to no change to thedQ/dV when additional CO₂ is added to a cell with NCA as the cathode.Without being bound by theory, CO₂ is produced from Li₂CO₃ on the NCAsurface, e.g., via the reaction: 2HF+Li₂CO₃=2LiF+H₂O+CO₂. Although someimplementations can use NCA as a cathode, various embodiments can useother materials (e.g., with CO₂) in place of NCA, such as othermaterials which may be safer, less expensive, and/or more readilyavailable than NCA.

Gas may negatively affect the cell performance including potentiallycausing high cell impedance. Certain embodiments described herein cangenerate CO₂ at a similar rate (or greater) of consumption (e.g. cellstend to gas at a fast rate) and achieve good cycle life. In addition, itcan be challenging to handle the gas, yet various implementationsdescribed herein can hold the cell layers together such that the gasdoes not degrade the cell.

Various embodiments have been described above. Although the inventionhas been described with reference to these specific embodiments, thedescriptions are intended to be illustrative and are not intended to belimiting. Various modifications and applications may occur to thoseskilled in the art without departing from the true spirit and scope ofthe invention as defined in the appended claims.

What is claimed is:
 1. A method of forming an electrochemical cell, themethod comprising: providing a first electrode, a second electrode, aseparator between the first and second electrodes, and an electrolyte ina cell container, wherein the first electrode comprises silicon-dominantelectrochemically active material, wherein the silicon-dominantelectrochemically active material comprises greater than 50% silicon byweight; exposing at least a part of the electrochemical cell to CO₂; andforming a solid electrolyte interphase (SEI) layer on the firstelectrode using the CO₂; wherein the first electrode, the secondelectrode, and the separator are held together with a polymer adhesiveon the separator in order to maintain adhesion between the firstelectrode, second electrode, and separator while the at least a part ofthe electrochemical cell is exposed to CO₂; wherein said SEI layer isabout 1 nm to about 20 nm thick and wherein the first electrode is aself-supported film.
 2. The method of claim 1, wherein the SEI layercomprises solid carbonate on LiF particles.
 3. The method of claim 1,wherein forming the SEI layer occurs during lithiation.
 4. The method ofclaim 1, wherein the silicon-dominant electrochemically active materialcomprises the silicon at about 60% to about 100% by weight.
 5. Themethod of claim 4, wherein the silicon-dominant electrochemically activematerial comprises the silicon at about 70% to about 100% by weight. 6.The method of claim 1, wherein the second electrode comprises nickelcobalt aluminum (NCA).
 7. The method of claim 6, wherein the CO₂ isproduced from Li₂CO₃ on a surface of the NCA.
 8. The method of claim 1,wherein the second electrode comprises lithium cobalt oxide (LCO),lithium manganese oxide (LMO), or nickel manganese cobalt (NMC).
 9. Themethod of claim 1, wherein the CO₂ is provided in the cell container.10. The method of claim 9, wherein the CO₂ is provided in an amount ofabout 0.01 to about 0.6 mg per mg of the silicon.
 11. The method ofclaim 10, wherein the CO₂ is provided in an amount of about 0.1 to about0.6 mg per mg of the silicon.
 12. The method of claim 9, wherein the CO₂is provided by adding dry ice to the electrolyte.
 13. The method ofclaim 1, wherein the CO₂ is dissolved in the electrolyte.
 14. The methodof claim 13, wherein the CO₂ is dissolved in an amount of about 0.01 toabout 0.6 mg per mg of the silicon.
 15. The method of claim 1, whereinthe CO₂ is produced from a carbonate in the cell container.
 16. Themethod of claim 1, wherein the electrolyte comprises greater than about10% to about 90% carbonate solvent by weight.
 17. The method of claim 1,wherein the electrolyte comprises a cyclic carbonate solvent.
 18. Themethod of claim 17, wherein the solvent comprises fluoroethylenecarbonate (FEC), difluoroethylene carbonate (F2EC), trifluoroethylenecarbonate (F3EC), ethylene carbonate (EC), propylene carbonate (PC),trifluoropropylene carbonate (TFPC), or a mixture thereof.
 19. Themethod of claim 1, wherein the electrolyte comprises a linear carbonatesolvent.
 20. The method of claim 19, wherein the solvent comprisesethylmethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate(DMC), methyl 2,2,2-trifluoroethyl carbonate (TFEMC), ethyl2,2,2-trifluoroethyl carbonate (TFDEC), difluoromethyl fluoromethylcarbonate (TFDMC), or a mixture thereof.
 21. The method of claim 1,wherein the separator comprises polyolefin, cellulose, or a combinationthereof.
 22. The method of claim 1, wherein the adhesive comprisespoly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/PMMA).
 23. Themethod of claim 1, wherein the first electrode is an anode and thesecond electrode is a cathode.
 24. The method of claim 1, wherein thefirst electrode comprises a Si-composite electrochemically active film.